Historical Forest Conditions within the Range of the Pacific Fisher and Spotted Owl in the Central and Southern Sierra Nevada, California, USA

Setting the Stage for Mixed- and High-Severity Fire

“…For example, Steel et al (2015, Table 7 in particular) modeled time since fire and fire severity in California's forests and predicted that, in mixed-conifer forests, high-severity fire would range from 12% 10 years after fire to 20% 75 years after fire, though the modeling for mixed-conifer forests seems to have been based on what appears to be very limited data for forests that experienced fire less than 75 years earlier (Steel et al, Figure 4), weakening inferences about a time since fire/severity relationship. Regardless, the high-severity fire values reported by Steel et al-even for forests that had not previously burned for 75-100 years-remain well within the range of natural variation of high-severity fire proportions in these forests found by most recent studies (Beaty and Taylor, 2001;Bekker and Taylor, 2001;Baker, 2014;Odion et al, 2014;Hanson and Odion, 2015a).…”

“…While this model fits reasonably well in some lowelevation, xeric forest systems (Perry et al, 2011;Williams andBaker, 2012a, 2013), it has been extrapolated far beyond where it seems to apply best. That higher fire severities occurred historically, albeit at a wide variety of spatial and temporal scales, in most or all fire-dependent vegetation types of western North America is becoming increasingly clear (Veblen and Lorenz, 1986;Mast et al, 1998;Taylor and Skinner, 1998;Brown et al, 1999;Kaufmann et al, 2000;Heyerdahl et al, 2001Heyerdahl et al, , 2012Wright and Agee, 2004;Veblen, 2006, 2007;Baker et al, 2007;Hessburg et al, 2007;Klenner et al, 2008;Amoroso et al, 2011;Perry et al, 2011;Schoennagel et al, 2011;Williams and Baker, 2012a;Marcoux et al, 2013;Odion et al, 2014;Hanson and Odion, 2015a).…”

“…Scientists have reviewed evidence in early historical US government forest reports, finding widespread occurrence of small and large higher-severity fire patches in all forest types, including ponderosa pine and mixed-conifer forests Hanson and Odion, 2015a). Evidence in these reports includes detailed descriptions of low-, mixed-, and highseverity fires; maps of slightly to severely burned forests; estimates of total area burned at mixed and high severity; and photographs of the landscapes after these fires.…”

“…A portion of the historical montane forest landscape in any given region undoubtedly comprised open forest dominated by low-severity fire (e.g., Brown et al, 1999, Fulé et al 2009, Iniguez et al, 2009Perry et al, 2011;Williams and Baker, 2012a;Hagmann et al, 2013;Baker, 2014), and some forest types (e.g., ponderosa pine [Pinus ponderosa]) often had a preponderance of low-severity fire in many low-elevation or xeric-type forest environments throughout western North America. Nevertheless, landscape-level evidence indicates that vast forested areas also comprised moderate to very dense forests characterized by a mixed-severity fire regime, wherein higher-severity fire patches of varying sizes occurred in a mosaic of low-and moderate-severity fire effects (Veblen andLorenz, 1986, 1991;Baker et al, 2007;Sherriff and Veblen, 2007;Hessburg et al, 2007;Perry et al, 2011;Baker, 2012;Williams and Baker, 2012a,b;Baker, 2014;Baker and Williams, 2015;Hanson and Odion, 2015a). In general, in historical ponderosa pine and mixed-conifer forests of the western United States, local variability was substantial (Brown et al, 1999, Fulé et al 2009, Iniguez et al, 2009Hessburg et al, 2007;Perry et al, 2011;Baker, 2012;Williams andBaker, 2012a,b, 2013;Baker, 2014;Baker and Williams, 2015;Hanson and Odion, 2015a).…”

“…Nevertheless, landscape-level evidence indicates that vast forested areas also comprised moderate to very dense forests characterized by a mixed-severity fire regime, wherein higher-severity fire patches of varying sizes occurred in a mosaic of low-and moderate-severity fire effects (Veblen andLorenz, 1986, 1991;Baker et al, 2007;Sherriff and Veblen, 2007;Hessburg et al, 2007;Perry et al, 2011;Baker, 2012;Williams and Baker, 2012a,b;Baker, 2014;Baker and Williams, 2015;Hanson and Odion, 2015a). In general, in historical ponderosa pine and mixed-conifer forests of the western United States, local variability was substantial (Brown et al, 1999, Fulé et al 2009, Iniguez et al, 2009Hessburg et al, 2007;Perry et al, 2011;Baker, 2012;Williams andBaker, 2012a,b, 2013;Baker, 2014;Baker and Williams, 2015;Hanson and Odion, 2015a). In sum, these and other studies indicate that historically there was high variability in fire effects (low to high severity) and composition and structure at both small and large spatial scales, and these patterns varied greatly depending on the regional and biophysical setting.…”